Development of microprocessor technology has seen decreasing power supply voltages that are more susceptible to interference by noise, decreasing signal transition times, decreasing die sizes, increasing power supply currents, and increasing clock speeds. As a result, ever more significant demands are placed upon the power supplies of microprocessor circuits. Such demands typically result in significant current variation between power and ground conductors.
Knowledge as to the nature of the variation in current from a power supply due to the operation of a microprocessor is useful for predicting signal integrity in a microprocessor. This is because the noise that is injected onto a power supply loop due to the change in the current demanded by the microprocessor is proportional to the rate of the change in the current. Consequently, actual knowledge as to the variation in the current can be important to verify the worst case system power supply loop response.
However, there are significant obstacles that prevent a direct measurement of current supplied to a microprocessor circuit. For example, the measurement of the supply current has to be performed while the microprocessor is under operating conditions. Under these conditions, the die and package are typically not separated, so a probe used for measurement can not be attached to the power and ground at the die. Also, equipment used to measure the current itself may further distort the current, thereby resulting in inaccurate results. As a result, it is difficult to measure actual current supplied to the die under operating conditions to confirm the accuracy of the modeling of such current during design of the microprocessor and the power supply.